WARNING: This page is about a different car, the 2003 GMC Safari and 2003 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC U1000 And U1255 Diagnostic Test (1 of 2)
Courtesy of GENERAL MOTORS CORP.
Fig 2: DTC U1000 And U1255 Diagnostic Test (2 of 2)